During processing of silver halide photographic materials, silver is often carried out of the photographic elements and into solutions, particularly the fix and bleach/fix solutions. As silver has more than a modest intrinsic value, many methods have been devised for desilvering these solutions to regain the metallic silver. Amongst the many available methods for reducing the silver in solution are electrolysis, contact reduction with a metal and chemical reduction. Each of these methods has its own advantages and disadvantages.
Electrolysis produces high purity silver because it reduces ionic silver by merely supplying electrons to the ions in solution. It requires a large initial capital expenditure and therefore reduces the profitability of silver recovery. Electrolysis is also ineffective at low concentrations of silver.
Chemical reduction and precipitation also can produce high purity silver, but the cost of the most effective reducing agents can be significant. The particle size of the reduced silver also can be quite small leading to difficulties in recovering the silver.
Metal contact reduction is an inexpensive means of reducing silver, but produces a low purity silver because of comingling with residual iron used in the oxidation/reduction process.
This metal displacement type of silver recovery process has traditionally been performed by passing silver rich solutions over iron, usually in the form of steel wool wherein an electron exchange occurs between the more and less noble metal. Iron is put into solution and the silver is allowed to settle out. The iron metal Fe.degree. is oxidized to ferrous or ferric ion and the silver ion Ag.sup.+ is reduced to silver metal. Some of the iron taken into solution is subsequently co-precipitated (by reduction) with the silver, which adulterates the purity of reclaimed noble metal. Because the iron or steel wool sits in a water based solution during this process and in a continuous wet condition during automatic machine processing, it is subject to attack due to oxidation or hydrogen ion effects.
When the stoichiometric balances between iron taken into solution and the amount of silver reduced were evaluated, it was found that in long term processsing where the silver rich solutions from automatic or manual film processors were not passed over the metal continuously (as with an overnight shut down), much more iron was being brought into solution than would be expected from the metal displacement silver recovery reaction, or even considering the normal oxidation of iron in an aqueous environment.
It has been found in the practice of the present invention that certain materials may be added to the steel wool in metal contact silver recovery processes which reduce the amount of iron brought into solution.
It has been found that the addition of certain materials to solutions and steel wool used in metal displacement silver recovery systems are especially effective in reducing the amount of iron which passes into solution during dormant periods of such processes.
Contact reduction (metal displacement) silver recovery has been described in prior art such as U.S. Pat. Nos. 3,630,505 and 3,369,801. Other techniques suggest the addition of certain materials to silver rich solutions which reduce the iron loss during active use of the contact reduction process by acting as reducing agents themselves and therefore changing the amount of iron necessary to reduce a given amount of silver. In a bleach fix bath, this last process is believed to work by converting NaFe.sup.(III) EDTA to Na.sub.2 Fe.sup.(II) EDTA and thereby eliminating the non-silver productive oxidative action of the first complex on steel wool and also the dissolving action of that complex on silver already deposited. This reduction of the complexed iron is effected by the addition of the chemical reducing agent.